Fall Armyworm Resistant to Bt Corn Found in North Carolina: Is Your Field Next?

Mr. Cam Kenimer, who runs our corn variety trials, sent me a link to an interesting article on armyworms developing resistance to Bt corn. The article, which is actually about fall armyworm (FAW), reports that this pest carries a gene that makes it resistant to the Bt trait Cry1F. This trait generally confers protection from caterpillar feeding in cotton (Widestrike™) and corn (Herculex 1™). This resistance has never before been reported further north than Louisiana or Florida. However, my colleague, Dr. Dominic Reisig, found FAW feeding and surviving on Cry1F Bt corn in North Carolina.

While Bt resistance is a serious concern, the fall armyworm we face in Kentucky is from a different population than the ones causing problems for North Carolina growers. Nevertheless, Bt-resistant fall armyworms may become a problem for us in the long run, so let’s take a closer look.

Fall Armyworm Migration Patterns

It pays to know a fundamental difference between the FAWs found in KY versus the ones in North Carolina. Fall armyworms can only survive the winter where its food plants grow all year round, which means FAW cannot survive northern winters. Moths migrate northward each spring from two very different sources: southern Florida and southern Texas.

The Florida population contains individuals that have a gene modification providing resistance to the Cry1F trait. The Texas population does not seem to have individuals with this modified gene. Why only Florida? This is likely because the Florida population interbreeds with populations in the Caribbean, which are the probable source of the resistance. South Florida and the Caribbean have year-round FAW populations and lots of use and testing of single trait corn products. It appears that the Texas population does not mix with the Florida population very much so it has not acquired the resistance gene.

The Texas population migrates north and east, up the Mississippi river valley into Kentucky, Tennessee, and the northern Corn Belt states. This Texas population does not carry the Cry1F resistance gene. The Florida population migrates out of south Florida and moves along the Gulf (of Mexico) Coast and northward along the southern and eastern sides of the Appalachian Mountains. Though there does seem to be something of a mixing area in Louisiana and Alabama, the Florida population does not appear able to get further west or north on that route. It does, however, move up the East Coast and eventually into New England and Canada. It look as if there may be some mixing of populations late in the season in eastern Pennsylvania or more northern states, but this is currently very unclear. The result of this separation in migration routes is that Kentucky is not exposed to the Florida FAWs  that carry the  resistance gene.

Figure 1. Fall army worm general migration routes and times of arrival in various parts of the U.S. (Graphic adapted from : Sparks, A. 1979. A Review of the Biology of the Fall Armyworm. Fla. Entomol. 62(2):82-87)

Figure 1. Fall army worm general migration routes and times of arrival in various parts of the U.S. (Graphic adapted from : Sparks, A. 1979. A Review of the Biology of the Fall Armyworm. Fla. Entomol. 62(2):82-87)

This is very good for Kentucky, but no one can know how long it will continue. The Kentucky IPM program has been sending FAW moths to USDA scientists in Gainesville, FL for several years. These moths are tested to determine from which population they originate. Thus far it has always been the Texas population. Nevertheless, the mixing zone in Louisiana has to remain a worry. What is keeping these Florida population moths from moving further west and north?  No one knows. But for now at least this is one problem that we don’t have.

References

Something Resistant This Way Comes: An Insect Mystery (NC State News) http://news.ncsu.edu/2014/11/resistant-insect-mystery/

Fangneng H., J. Qureshi, R. Meagher, Jr., D. Resig, G. Head, D. Andow, X.Ni, D. Kerns, G.D. Buntin , Y. Niu, F. Yang, and V. Dangal.2014. Cry1F Resistance in Fall Armyworm Spodoptera frugiperda: Single Gene verses Pyramided Bt Maize. Plos One. DOI:10.1371/journal.prone.0112958.

 

by Doug Johnson, Extension Entomologist

 

 

Posted in Grains